Small internal combustion engines, especially two-stroke and four-stroke spark ignition engines, are used to provide power to a variety of machinery, e.g., gasoline-powered lawn mowers, chain saws, leaf blowers, string cutters, leaf blowers, motor scooters, motorcycles and the like. Such engines provide a severe environment for a catalytic exhaust treatment apparatus. This is due to the fact that in small engines, the exhaust gas contains a high concentration of unburned fuel and unconsumed oxygen.
Additionally, the vibrational force in a two-stroke engine can be three or four times that of a four-stroke engine. For example, vibrational accelerations of 70 to 90 gravitational acceleration at 150 Hertz have been reported for small internal combustion engines. The harsh vibration and exhaust gas temperature conditions associated with small internal combustion engines lead to several modes of failure in the exhaust gas catalytic treatment apparatus, including failure of the mounting structure by which a catalyst member is secured in the apparatus and consequential damage or destruction of the catalyst member due to the mechanical vibration and to flow fluctuation of the exhaust gas under high temperature conditions.
The catalyst member usually comprises a ceramic-like carrier member typically made of e.g., cordierite, mullite, etc., on which an exhaust treatment catalytic material is coated. The ceramic-like material is subject to cracking and pulverization due to excessive vibration. Metal carrier members, i.e., metal substrates, are obvious replacements for the ceramic-like materials, but have their own problems as brought out below.
In the near future, small internal combustion engines will become subject to stringent federal and state emission control regulations. Accordingly, these small engines will require exhaust emission control systems analogous to those currently employed for control of automotive emissions.
For the exhaust emission control systems to be used for the small internal combustion engines typically a metal substrate (also referred to in the prior art as a metal carrier member) will be coated with an exhaust control treatment catalyst such as a three-way conversion (“TWC”) catalyst that will control the emissions of hydrocarbons, carbon monoxide and nitrogen oxides. The challenge has been to impart the required physical properties to the metal substrate that will permit the catalyst to withstand high temperature conditions with severe vibration and poisons from oil and ash over a wide variety of small engine platforms with limited lives and the need for such small engine platforms to operate under a wide range of air/fuel ratios and space velocities. Major differences in the coefficients of thermal expansion of a precious metal catalyst and the metal substrate exacerbate this problem.
The exhaust control emission catalyst materials are typically used in particulate form with particles in the micron-sized range, e.g., 10 to 20 microns in diameter, so that they can be formed into a slurry and applied as a washcoat on the carrier member. Known TWC catalysts that exhibit good activity and long life comprise one or more platinum group metals (e.g., platinum or palladium, rhodium, ruthenium and iridium) disposed on a high surface area, refractory oxide support, e.g., a high surface area alumina coating. The principal problem associated with the coating of metal substrates with an exhaust emission control treatment catalyst such as a TWC catalyst is adherence of the catalyst to the metal substrate over the wide range of conditions to which the catalyst-coated metal substrate will be exposed during operation of the small engine platform.